Regenerative optical pumping system



.iw @www REGENRTEVE OPTXCL PUMPING SYSTEM Fied May a, 1962 NVENTOV MTORNEMJ 3,27L695 REGENIERATWE OPTlCAlL lill-MPN@ EllS'lllllMl lohn W. Marini, Washington., DE., assigner to lliitton Systems, ilnc.,, College illarlr, Md. Filed Miay il, l962,-Ser. No. 1%,lj 5 Claims. (El, 33t-94.5)

This invention relates to improvements in the optical pumping of microwave resonant mediums and more particularly is directed to improvements in the coupling of the resonant medium to a source of circularly polarized photon energy to provide regenerative application of the photon energy. l

lt is accordingly'a principal object of the invention to provide an improved manner of coupling circularly polarized photon energy to a microwave resonant material.

Stated in another manner, it is an object of the invention to obtain greater energy Vtransfer between a source of circularly polarized photon energy and-a resonant medium responsive to such energy thereby` to reduce thel intensity ot the source of photon energy while transf ferring comparable energy to the medium, or alternatively obtaining a greater transfer of energy to the medium with the same intensity of photon radiation than was heretofore obtainable.

Dptical pumping is a term -that has been applied to the process of imparting energy to and orienting the atoms of certainresonant mediums, such as the alkali nretal vapors, by the use of photon radiation, at frequencies that are resonant with that of the medium. In recent years optically pumped mediums have been ernployed in microwave amplifiers, oscillators atomic clocks, and magnetometers among many others. @ne of 'the serious limitations in the more widespread uses of such devices is the weight, size and intensity requirements of the source of photon radiation which prohibits the use of such apparatus for many portable applications, such as various aircraft uses, where lightweight and small size are premium considerations. Another limitation preventing various desirable uses is the unavailability of suil ciently intense sources of circularly polarized photon radiation that are required for many applications.

Very generally according to the present invention, there is provided an improved apparatus for optical pumping employingthe regenerative application of circularly polarized photon radiation to the medium. Considered in one manner, this is perfor-med by repetitively directing the radiation to successively traverse the resonant medium; and considered in a more electrical sense, this is performed by producing a standing wave of photon radiation in the medium. By this process a considerably more efficient transfer of energy between the circularly polarized radiation andthe medium is obtained permitting considerable reductions in the intensity, and hence in the size and weight of the source of photon radiation. Alternatively using availabe light sources, a 'greater transfer of energy may be obtained than heretofore, equivalent to that formerly provided by a more intense light source.

ther objects and additional advantages will be more readily appreciated by those skilled in the art after a detailed consideration of the following specification taken with the accompanying drawing.

Referring to the drawing, there is. generally shown a transparent absorption cell lli containing a resonant mediurn Ill such as a mixture of a resonantrgas and a buiier gas; a lamp or source l2 for producing photon radiation at the frequency of the resonant medium, and an optical system for producing circularly polarized radiation and regeneratively applying this radiation to optically pump the medium ll.

The gas employed within cell may be a rubidium Cil . 'i Patenten vapor at a pressure of the order of lil-6 rnrn. ormercury Vtogether with a buffer gas, such as argon at about 3U mm.

of mercury. However, as known in the art sodium vapor, cesium or other alkali metal vapors may be employed as well as other butler gases such as neon, helium or the like. A lowppressurc of rubidium vapor is used for minimizing relaxation due to rubidium-rubidium collisions. The buffer gas serves to inhibit collisions ot' the rubidium with the cell walls whereasthe rubidium-butfer gas collisions do not cause appreciable relaxation or disorientation of the rubidium atoms.

The lamp l2 may be a rubidium vapor lamp to pro vide D1 resonance radiation at 794.7 millimicrons, or a sodium lamp, cesium lamp or other may be employed corresponding to the resonant medium lll selected for the absorption cell lil.

Unpolarized rays 2l from the lamp l2 are initially focused by a lens llf to provide parallel rays along the axis of the System. An interference filter lul is employed to remove the D2 line at a, frequency of 780 millimicrons and a combination of a linear polarizer plate lo and a quarter wave plate are next employed in sequence along the path of thev photon radiation 2l to produce circularly polarized radiation for illuminating the resonant medium within the transparent cell lll.

VAccording to the invention, there is mso applied a wealc magnetic iield (not shown) directed along the axis of the light rays so that the selection rule AMF=+l is operative. The reason underlying this selection rule is that each photon of circularly polarized light carries one unit of angular momentum that must be acquired by an atom of the gas interacting with the photon. After applying the magnetic field and iight radiation for a given interval 'the atoms of the gas become polarized with a net angular momentum and an associated magnetic moment along the direction of the pumping light and the applied magnetic lield. Accordingly, the gas lll is optionally pumped and absorbs a reduced amount of resonance radiation.

Normally', in accordance with prior art practices, each photon of the circularly polarized pumping light from the lamp l?. passes through the absorption cell only once and the transfer of energy between the photon radiation and the resonant medium is therefore accomplished by ymeans of a traveling wave of circularly polarized electromagnetic energy. According to the present invern tion, there is provided a process for regeneratively apply ing this radiation tothe medium thereby to more eliciently couple the radiation to the medium.

Returning to the drawing1 one preferred manner of performing this regenerative application ,of light energy is passing the light through a pair of interferometer plates i7 and its disposed in parallel on opposite sides of the cell lli. As shown, the first plate l?" is preferably cornprised ot a transparent planar member having a half silvercd reflecting surface fr@ racing the lcell ill on one side thereof and the opposite plate 'tti is provided with a fully mirrored surface l@ facing the opposite side ol the cell liti and accurately parallel with the reflecting surface fr@ of the first plate ll/ to repctitivcl y rcticct light between the two mirror surfaces. These confronting mirrors provide what may oe considered as an optically resonant cavity about the cel-l lll to produce a standing wave of light energy therein, produced by light interference, instead of a traveling wave oi' photon electromagnetic energy as heretofore.

in operation, the circularly polarized light leaving the quarter wave plate lo passes through the half siivered mirror surface 2@ and thence through the resonant mc um .ll where some of the energy is absorbed while a maining portion reaches the mirror surface t #l nl@ surface t9, this remaining portion is reflected backwardly and regcneratively applied to the medium where a portion thereof is again absorbed while the remainder reaches the iirstmirror surface 20. A light interference pattern is, therefore, established to producev standing waves of photon radiation within the resonant medium ill. These standing waves impart considerably greater energy to the :medium than heretofore to enhance the pumping rate and efeiency of the system.

. The dimensions of the mirror or reflecting surfaces in this arrangement are about a's large as the distance between these surfaces thereby to accommodate a rather large absorption cell 10. For minimizing any undesirable light. interaction between the walls of the absorption cell and the interferometer surfaces, the interferometer plates may alternatively be placed inside the cell 1U (not shown). Alternatively, the absorption cell itself may be formed with a pai-r of parallel walls provided with the mirror surfaces 19 and 20 to function as the interferometer plates (not shown).

Many other variations mayalso be made without de' parting from the spirit and scope of this' invention and accordingly this invention should be considered as Ibeing limited only according to the following claims.

What is claimed is:

l. in an optical pumpingsystem including an optical vabsorption cell'co'ntaining a gas resonant at microwave radio frequencies, and including means for producing photon radiation at the resonant frequency of the gas, circularly polarizing said radiation, and illuminating the gas thereby to couple energy thereto, means for applying a magnetic eld to the gas, the improvement comprising means for regeneratively applying the photonra'diation to the gas to traverse the gas more than twice and increase the transfer of energy thereto, said means including an optical interferometer.

2. In the system of claim 1, said interfereometcr comprising a'pairof radiation reectors disposed outside the absorption cell and in confronting relationship,

and means for introducing the photon radiation between the reflectors to successively traverse the gas.

'3; In the system of-clairn 1, said interferometer cornprising a pair of radiation reilectingmeans disposed with respect to the cell and with respect to the photon radia tion to produce standing waves of the radiation within the gas.

, if fi. in the system of claim L sad interferometer cornprising' apair of radiation reflector means in confronting relationship with thel resonant gas between,

the reflecting means having ay reflecting surface of dimensions approximating the distance. between the surfaces thereby to accommodate a large absorption cell. v S. An optical pumping system comprising: an absorption cell containing a microwave resonant medium and being transparent to photon radiation at the resonant frequency of the medium, an optical system being'energizable by'a source of photon radiation at the resonant frequency of the medium, and comprising means for producing circularly polarized photon radiation at that frequency, an interferometer being illuminated by said circularly -polarized radiation for regeneratively applying the radiation to the absorption cell, said interferometer providing a vresonant cavity for the photon radiation. at the resonant frequency thereby to produce standing waves of circularly polarized radiation within said medium to traverse said medium more than twice in order to increase the photon energy transfer to said medium.

References Cited by the Examiner UNlTED STATES PATENTS 2,884,524 4/1959 Dicke SSS-1 3,098,112 7/1963 Horton 88--61 3,211,055 10/1965 Andres 88v6l OTHER REFERENCES Boyle et al.: A Continuously Operating Ruby Optical Maser, Applied Optics, vol. l, No. 2, March i962.

Kastler: Atomes a linterieur dun interferometer Perot-Fabry, Applied Optics, vol. l, No. l., anuary i962, pp. 17-24; pp. 22-24 specifically relied upon.

rEwELL H. PEDERSEN, Primary Examiner.

DAVE) H. RUBIN, Examiner. R. L. WBERT, Assistant Examiner. 

5. AN OPTICAL PUMPING SYSTEM COMPRISING: AN ABSORPTION CELL CONTAINING A MICROWAVE RESONANT MEDIUM AND BEING TRANSPARENT TO PHOTON RADIATION AT THE RESONANT FREQUENCY OF THE MEDIUM, AN OPTICAL SYSTEM BEING ENERGIZABLE BY A SOURE OF PHOTON RADIATION AT THE RESONANT FREQUENCY OF THE MEDIUM, AND COMPRISING MEANS FOR PRODUCING CIRCULARLY POLARIZED PHOTON RADIATION AT THAT FREQUENCY, AN INTERFEROMETER BEING ILLUMINATED BY SAID CIRCULARLY POLARIZED RADIATION FOR REGENERATIVELY APPLYING THE RADIATION TO THE ABSORPTION CELL, SAID INTERFEROMETER PROVIDING A RESONANT CAVITY FOR THE PHOTON RADIATION AT THE RESONANT FREQUENCY THEREBY TO PRODUCE STANDING WAVES OF CIRCULARLY POLARIZED RADIATION WITHIN SAID MEDIUM TO TRAVERSE SAID MEDIUM MORE THAN TWICE IN ORDER TO INCREASE THE PHOTON ENERGY TRANSFER TO SAID MEDIUM. 